This invention pertains to silver halide photographic materials, in particular to color photographic materials having reduced granularity achieved by incorporating a novel carbamic acid solubilized smearing coupler into the material.
Photographic coatings incorporating color couplers and light-sensitive silver halide emulsions have been known for many years. Generally, such couplers include ballast groups of sufficient size to immobilize both the coupler and a dye formed from the coupler on reaction with oxidized color developing agent during development. The corresponding silver image formed is then bleached and removed by a fixing bath to leave a colored image composed only of dye. Materials of higher photographic speed have required the use of larger silver halide grains which results in a color image formed from larger dye clouds. This has led in many cases to an undesirable grainy appearance. A physical measurement of such graininess is termed granularity. Granularity is due to the formation of dye deposits only in the immediate area of the silver grain where oxidized developer is formed, thus creating micro regions of high and low density.
U.S. Pat. No. 4,420,556 to Booms et al. describes the usefulness of photographic dyes which have the ability to diffuse a small distance from their generation site, that is, smear, and thus increase covering power and reduce granularity. Covering power is the density produced by a fixed amount of dye per unit area. The couplers described are two-equivalent couplers in which the primary ballast is attached to a part of the coupler moiety that does not form a dye upon reaction with oxidized developer. The ballast hinders or prevents diffusion before development, but upon development the ballast groups are detached and the resulting dye is free to diffuse through the film. This results in good granularity because the diffusion will tend to smooth out micro density variations. However, once the ballast is lost, diffusion is only controlled by the nature of the substituents left on the dye itself. Booms et al. attach a secondary ballast to the coupler to render the dye slightly mobile during development. However, if the diffusion, or smearing, continues even after development because the dye remains mobile, there will be an undesirable loss in image sharpness with time. This is because sharpness is a function of the gradient between two closely spaced regions of high and low density, that is, an edge. Post-process dye diffusion will allow dye to move from high to low density regions, reducing the gradient and will lead to total loss of image structure with time.
A second embodiment of the Booms et al. patent involves incorporating a coupler yielding a diffusible dye, and controlling smearing by immobilizing this dye on a nearby mordant before it diffuses too great a distance. The degree of smearing is controlled by positioning of the mordant a certain distance from the color coupler. The greater the distance, the greater will be the degree of image smearing.
Additional documents relating to reducing graininess through the use of dye smearing include U.K. Patent Application 2,141,250 which achieves increased sensitivity and improvements in granularity with the use of smearing couplers with silver halide emulsions that have an average size above 1.5 micrometers. U.S. Pat. No. 4,840,884 discloses couplers that release a shifted azoaniline dye as a carbamic acid derivative upon reaction with oxidized developer. The carbamic acid group is not stable and decomposes later in the process to give carbon dioxide and unshifted azoaniline dye. U.S. Pat. No. 4,489,155 describes the use of couplers which yield somewhat diffusible dyes where the size of the dye cloud is limited by including competitors which scavenge oxidized developer. A similar result is obtained using combinations of immobile dye-producing high activity couplers with couplers which yield diffusible dyes, as disclosed in U.S. Pat. No. 4,567,135. European Patent 96,873 seeks improved granularity and sharpness by employing matched activity combinations of DIR couplers with couplers producing controlled smearing dyes. U.S. Pat. Nos. 4,536,472; 4,705,743; and 4,729,944, and European Patent Applications 135,883 and 230,975 examine combinations of controlled smearing dye couplers with silver halide emulsions of various descriptions. U.S. Pat. No. 5,051,343 relates to couplers that are removed from a film element if they do not undergo reaction with oxidized developer.
Solubilizing substituents that are at least partly ionizable in the developer such as carboxylic acids or sulfonamides allow for good diffusion during development, but do not prevent continued smearing after processing is completed. Alternatively, if the dye has no ionizable groups, then the rate of post-processing smearing can be acceptable, but the amount of smearing during development is also low. This limits the amount of the granularity improvement available from the use of this type of coupler. Accordingly, a material that contains a good solubilizing group in the developer to give good diffusion, but has no solubilization after the process is complete to prevent post-process diffusion, would be highly desirable.
Thus, there has been a need to obtain diffusion of the dye during development, but to prevent dye diffusion after processing is complete, in order to avoid loss of image structure over time. It would therefore be highly desirable to provide couplers, and photographic elements containing them, wherein diffusion of the coupler is prevented or limited before development, wherein during development the resulting dye is free to diffuse, that is, smears to reduce granularity, but wherein after processing is complete, further smearing of the image is greatly reduced so as not to undesirably reduce image sharpness.